<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<input type="text"  id="text_a" />
		<select name="chose" id="s_chose">
			<option value="a">+</option>
			<option value="b">-</option>
			<option value="c">x</option>
			<option value="d">/</option>
		</select>
		<input type="text"  id="text_b" />
		结果：
		<input type="text"  id="text_res"/>
		<button id="b_calc" >计算</button>
	</body>
	<script type="text/javascript">
		document.getElementById("b_calc").onclick=function(){
		var chose=document.getElementById("s_chose").value
		var text_a=parseFloat(document.getElementById("text_a").value)
		var text_b=parseFloat(document.getElementById("text_b").value)
			if(chose=="a"){
				document.getElementById("text_res").value= (text_a+text_b).toFixed(4);
			}else if (chose=="b"){
				document.getElementById("text_res").value= (text_a-text_b).toFixed(4);
			}else if (chose=="c"){
				document.getElementById("text_res").value= (text_a*text_b).toFixed(4);
			}else if (chose=="d"){
				document.getElementById("text_res").value= (text_a/text_b).toFixed(4);
			}
		}
		
	</script>
</html>
